1879.] NATURAL SCIENCES OP PHILADELPHIA. 355 



When Dr. White proposed the genus Cceliocrinus, he had evi- 

 dently overlooked De Koninck's genus Hydreionocrinus, with 

 which it agrees in all essential points except in the form of the 

 ventral sac, which is balloon-shaped in Cceliocrinus, instead of 

 like a mushroom, and except also that in Hydreionocrinus the 

 calyx is comparatively larger and the arms shorter. Whether these 

 differences are of sufficient importance to justify even a suhgeneric 

 division, is to us somewhat doubtful. The two forms are certainly 

 not generically distinct, for they shade into one another so closely 

 that in some species it becomes difficult even to separate them on 

 the above characters. We, for our part, are inclined to recognize 

 in Cceliocrinus, which occurs in the lower Subcarboniferous, the 

 earlier stage of Hydreionocrinus; the latter, which nourished 

 during the later periods of the Carboniferous, both in Europe and 

 America, representing the culmination of the form in maturity 

 and extravagance, as developed in the course of time. Such ex- 

 traordinary development in certain parts of the animal as here 

 in the ventral sac frequently occurs when a genus is about to 

 become extinct, and Hydreionocrinus is, in fact, one of the last 

 survivors of this family. Bearing this in mind, it is of little 

 consequence whether we place the species of this gi-oup under 

 Hydreionocrinus, or subgenerically under Cceliocrinus. 



The relations of this form with Zeacrinus are also very close 

 and interesting, and the question may well be asked whether all 

 three forms ought not to be placed under Woodocrinus, provided 

 this genus possesses an inflated sac, which we consider the most 

 characteristic feature of this little group; especially as they 

 further agree in the shortness of the arms and their peculiar mode 

 of bifurcation. Such a classification would certainly be more 

 natural than to place them under Poteriocrinus, as was done by 

 Hall, and subgenerically by Meek and Worthen. All Poterio- 

 crini have long arms, and the ventral sac is prolonged into a 

 slender tube. The arms in all species of Cceliocrinus are com- 

 posed of strongly wedge-shaped plates, which almost interlock, as 

 they do completely in most species of Hydreionocrinus, while the 

 corresponding joints in Zeacrinus and Woodocrinus are regularly 

 quadrangular, with parallel sutures. 



Geological position, etc. Found only in the lower part of the 

 Subcarboniferous of America. 



